constify from_fn, try_from_fn, try_map, map

library/core/src/array/drain.rs

@@ -1,76 +1,61 @@
-use crate::iter::{TrustedLen, UncheckedIterator};
+use crate::assert_unsafe_precondition;
+use crate::marker::Destruct;
 use crate::mem::ManuallyDrop;
-use crate::ptr::drop_in_place;
-use crate::slice;
 
-/// A situationally-optimized version of `array.into_iter().for_each(func)`.
-///
-/// [`crate::array::IntoIter`]s are great when you need an owned iterator, but
-/// storing the entire array *inside* the iterator like that can sometimes
-/// pessimize code.  Notable, it can be more bytes than you really want to move
-/// around, and because the array accesses index into it SRoA has a harder time
-/// optimizing away the type than it does iterators that just hold a couple pointers.
-///
-/// Thus this function exists, which gives a way to get *moved* access to the
-/// elements of an array using a small iterator -- no bigger than a slice iterator.
-///
-/// The function-taking-a-closure structure makes it safe, as it keeps callers
-/// from looking at already-dropped elements.
-pub(crate) fn drain_array_with<T, R, const N: usize>(
-    array: [T; N],
-    func: impl for<'a> FnOnce(Drain<'a, T>) -> R,
-) -> R {
-    let mut array = ManuallyDrop::new(array);
-    // SAFETY: Now that the local won't drop it, it's ok to construct the `Drain` which will.
-    let drain = Drain(array.iter_mut());
-    func(drain)
+#[rustc_const_unstable(feature = "array_try_map", issue = "79711")]
+#[unstable(feature = "array_try_map", issue = "79711")]
+pub(super) struct Drain<'a, T, U, const N: usize, F: FnMut(T) -> U> {
+    array: ManuallyDrop<[T; N]>,
+    moved: usize,
+    f: &'a mut F,
 }
-
-/// See [`drain_array_with`] -- this is `pub(crate)` only so it's allowed to be
-/// mentioned in the signature of that method.  (Otherwise it hits `E0446`.)
-// INVARIANT: It's ok to drop the remainder of the inner iterator.
-pub(crate) struct Drain<'a, T>(slice::IterMut<'a, T>);
-
-impl<T> Drop for Drain<'_, T> {
-    fn drop(&mut self) {
-        // SAFETY: By the type invariant, we're allowed to drop all these.
-        unsafe { drop_in_place(self.0.as_mut_slice()) }
+#[rustc_const_unstable(feature = "array_try_map", issue = "79711")]
+#[unstable(feature = "array_try_map", issue = "79711")]
+impl<T, U, const N: usize, F> const FnOnce<(usize,)> for &mut Drain<'_, T, U, N, F>
+where
+    F: [const] FnMut(T) -> U,
+{
+    type Output = U;
+
+    extern "rust-call" fn call_once(mut self, args: (usize,)) -> Self::Output {
+        self.call_mut(args)
     }
 }
-
-impl<T> Iterator for Drain<'_, T> {
-    type Item = T;
-
-    #[inline]
-    fn next(&mut self) -> Option<T> {
-        let p: *const T = self.0.next()?;
-        // SAFETY: The iterator was already advanced, so we won't drop this later.
-        Some(unsafe { p.read() })
-    }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        let n = self.len();
-        (n, Some(n))
+#[rustc_const_unstable(feature = "array_try_map", issue = "79711")]
+#[unstable(feature = "array_try_map", issue = "79711")]
+impl<T, U, const N: usize, F> const FnMut<(usize,)> for &mut Drain<'_, T, U, N, F>
+where
+    F: [const] FnMut(T) -> U,
+{
+    extern "rust-call" fn call_mut(&mut self, (i,): (usize,)) -> Self::Output {
+        // SAFETY: increment moved before moving. if `f` panics, we drop the rest.
+        self.moved += 1;
+        assert_unsafe_precondition!(
+            check_library_ub,
+            "musnt index array out of bounds", (i: usize = i, size: usize = N) => i < size
+        );
+        // SAFETY: caller guarantees never called with number >= N (see `Drain::new`)
+        (self.f)(unsafe { self.array.as_ptr().add(i).read() })
     }
 }
-
-impl<T> ExactSizeIterator for Drain<'_, T> {
-    #[inline]
-    fn len(&self) -> usize {
-        self.0.len()
+#[rustc_const_unstable(feature = "array_try_map", issue = "79711")]
+#[unstable(feature = "array_try_map", issue = "79711")]
+impl<T: [const] Destruct, U, const N: usize, F: FnMut(T) -> U> const Drop
+    for Drain<'_, T, U, N, F>
+{
+    fn drop(&mut self) {
+        let mut n = self.moved;
+        while n != N {
+            // SAFETY: moved must always be < N
+            unsafe { self.array.as_mut_ptr().add(n).drop_in_place() };
+            n += 1;
+        }
     }
 }
-
-// SAFETY: This is a 1:1 wrapper for a slice iterator, which is also `TrustedLen`.
-unsafe impl<T> TrustedLen for Drain<'_, T> {}
-
-impl<T> UncheckedIterator for Drain<'_, T> {
-    unsafe fn next_unchecked(&mut self) -> T {
-        // SAFETY: `Drain` is 1:1 with the inner iterator, so if the caller promised
-        // that there's an element left, the inner iterator has one too.
-        let p: *const T = unsafe { self.0.next_unchecked() };
-        // SAFETY: The iterator was already advanced, so we won't drop this later.
-        unsafe { p.read() }
+impl<'a, T, U, const N: usize, F: FnMut(T) -> U> Drain<'a, T, U, N, F> {
+    /// SAFETY: must be called without indexing out of bounds. (see `Drain::call_mut`)
+    // FIXME(const-hack): this is a hack for `let guard = Guard(array); |i| f(guard[i])`.
+    pub(super) const unsafe fn new(array: [T; N], f: &'a mut F) -> Self {
+        Self { array: ManuallyDrop::new(array), moved: 0, f }
     }
 }

library/core/src/array/mod.rs

@@ -12,6 +12,7 @@ use crate::fmt;
 use crate::hash::{self, Hash};
 use crate::intrinsics::transmute_unchecked;
 use crate::iter::{UncheckedIterator, repeat_n};
+use crate::marker::Destruct;
 use crate::mem::{self, MaybeUninit};
 use crate::ops::{
     ChangeOutputType, ControlFlow, FromResidual, Index, IndexMut, NeverShortCircuit, Residual, Try,
@@ -24,7 +25,6 @@ mod drain;
 mod equality;
 mod iter;
 
-pub(crate) use drain::drain_array_with;
 #[stable(feature = "array_value_iter", since = "1.51.0")]
 pub use iter::IntoIter;
 
@@ -104,9 +104,10 @@ pub fn repeat<T: Clone, const N: usize>(val: T) -> [T; N] {
 /// ```
 #[inline]
 #[stable(feature = "array_from_fn", since = "1.63.0")]
-pub fn from_fn<T, const N: usize, F>(f: F) -> [T; N]
+#[rustc_const_unstable(feature = "const_array", issue = "147606")]
+pub const fn from_fn<T: [const] Destruct, const N: usize, F>(f: F) -> [T; N]
 where
-    F: FnMut(usize) -> T,
+    F: [const] FnMut(usize) -> T + [const] Destruct,
 {
     try_from_fn(NeverShortCircuit::wrap_mut_1(f)).0
 }
@@ -142,11 +143,13 @@ where
 /// ```
 #[inline]
 #[unstable(feature = "array_try_from_fn", issue = "89379")]
-pub fn try_from_fn<R, const N: usize, F>(cb: F) -> ChangeOutputType<R, [R::Output; N]>
+#[rustc_const_unstable(feature = "array_try_from_fn", issue = "89379")]
+pub const fn try_from_fn<R, const N: usize, F>(cb: F) -> ChangeOutputType<R, [R::Output; N]>
 where
-    F: FnMut(usize) -> R,
-    R: Try,
-    R::Residual: Residual<[R::Output; N]>,
+    F: [const] FnMut(usize) -> R + [const] Destruct,
+    R: [const] Try<Residual: Residual<[R::Output; N]>>,
+    R::Output: [const] Destruct,
+    <R::Residual as Residual<[R::Output; N]>>::TryType: [const] Try,
 {
     let mut array = [const { MaybeUninit::uninit() }; N];
     match try_from_fn_erased(&mut array, cb) {
@@ -542,9 +545,11 @@ impl<T, const N: usize> [T; N] {
     /// ```
     #[must_use]
     #[stable(feature = "array_map", since = "1.55.0")]
-    pub fn map<F, U>(self, f: F) -> [U; N]
+    #[rustc_const_unstable(feature = "const_array", issue = "147606")]
+    pub const fn map<F, U>(self, f: F) -> [U; N]
     where
-        F: FnMut(T) -> U,
+        F: [const] FnMut(T) -> U + [const] Destruct,
+        U: [const] Destruct,
     {
         self.try_map(NeverShortCircuit::wrap_mut_1(f)).0
     }
@@ -580,11 +585,19 @@ impl<T, const N: usize> [T; N] {
     /// assert_eq!(c, Some(a));
     /// ```
     #[unstable(feature = "array_try_map", issue = "79711")]
-    pub fn try_map<R>(self, f: impl FnMut(T) -> R) -> ChangeOutputType<R, [R::Output; N]>
+    #[rustc_const_unstable(feature = "array_try_map", issue = "79711")]
+    pub const fn try_map<R, F>(self, mut f: F) -> ChangeOutputType<R, [R::Output; N]>
     where
-        R: Try<Residual: Residual<[R::Output; N]>>,
+        F: [const] FnMut(T) -> R + [const] Destruct,
+        R: [const] Try<Residual: Residual<[R::Output; N]>>,
+        R::Output: [const] Destruct,
+        <R::Residual as Residual<[R::Output; N]>>::TryType: [const] Try,
     {
-        drain_array_with(self, |iter| try_from_trusted_iterator(iter.map(f)))
+        // SAFETY: try_from_fn calls `f` with 0..N.
+        let mut f = unsafe { drain::Drain::new(self, &mut f) };
+        let out = try_from_fn(&mut f);
+        mem::forget(f); // it doesnt like being remembered
+        out
     }
 
     /// Returns a slice containing the entire array. Equivalent to `&s[..]`.
@@ -878,12 +891,15 @@ where
 /// not optimizing away.  So if you give it a shot, make sure to watch what
 /// happens in the codegen tests.
 #[inline]
-fn try_from_fn_erased<T, R>(
+#[rustc_const_unstable(feature = "array_try_from_fn", issue = "89379")]
+const fn try_from_fn_erased<T, R, F>(
     buffer: &mut [MaybeUninit<T>],
-    mut generator: impl FnMut(usize) -> R,
+    mut generator: F,
 ) -> ControlFlow<R::Residual>
 where
-    R: Try<Output = T>,
+    T: [const] Destruct,
+    R: [const] Try<Output = T>,
+    F: [const] FnMut(usize) -> R + [const] Destruct,
 {
     let mut guard = Guard { array_mut: buffer, initialized: 0 };
 
@@ -923,7 +939,8 @@ impl<T> Guard<'_, T> {
     ///
     /// No more than N elements must be initialized.
     #[inline]
-    pub(crate) unsafe fn push_unchecked(&mut self, item: T) {
+    #[rustc_const_unstable(feature = "array_try_from_fn", issue = "89379")]
+    pub(crate) const unsafe fn push_unchecked(&mut self, item: T) {
         // SAFETY: If `initialized` was correct before and the caller does not
         // invoke this method more than N times then writes will be in-bounds
         // and slots will not be initialized more than once.
@@ -934,11 +951,11 @@ impl<T> Guard<'_, T> {
     }
 }
 
-impl<T> Drop for Guard<'_, T> {
+#[rustc_const_unstable(feature = "array_try_from_fn", issue = "89379")]
+impl<T: [const] Destruct> const Drop for Guard<'_, T> {
     #[inline]
     fn drop(&mut self) {
         debug_assert!(self.initialized <= self.array_mut.len());
-
         // SAFETY: this slice will contain only initialized objects.
         unsafe {
             self.array_mut.get_unchecked_mut(..self.initialized).assume_init_drop();

library/core/src/ops/try_trait.rs

@@ -1,3 +1,4 @@
+use crate::marker::{Destruct, PhantomData};
 use crate::ops::ControlFlow;
 
 /// The `?` operator and `try {}` blocks.
@@ -380,17 +381,37 @@ pub(crate) type ChangeOutputType<T: Try<Residual: Residual<V>>, V> =
 /// Not currently planned to be exposed publicly, so just `pub(crate)`.
 #[repr(transparent)]
 pub(crate) struct NeverShortCircuit<T>(pub T);
+// FIXME(const-hack): replace with `|a| NeverShortCircuit(f(a))` when const closures added.
+pub(crate) struct Wrapped<T, A, F: FnMut(A) -> T> {
+    f: F,
+    p: PhantomData<(T, A)>,
+}
+#[rustc_const_unstable(feature = "const_never_short_circuit", issue = "none")]
+impl<T, A, F: [const] FnMut(A) -> T + [const] Destruct> const FnOnce<(A,)> for Wrapped<T, A, F> {
+    type Output = NeverShortCircuit<T>;
+
+    extern "rust-call" fn call_once(mut self, args: (A,)) -> Self::Output {
+        self.call_mut(args)
+    }
+}
+#[rustc_const_unstable(feature = "const_never_short_circuit", issue = "none")]
+impl<T, A, F: [const] FnMut(A) -> T> const FnMut<(A,)> for Wrapped<T, A, F> {
+    extern "rust-call" fn call_mut(&mut self, (args,): (A,)) -> Self::Output {
+        NeverShortCircuit((self.f)(args))
+    }
+}
 
 impl<T> NeverShortCircuit<T> {
     /// Wraps a unary function to produce one that wraps the output into a `NeverShortCircuit`.
     ///
     /// This is useful for implementing infallible functions in terms of the `try_` ones,
     /// without accidentally capturing extra generic parameters in a closure.
     #[inline]
-    pub(crate) fn wrap_mut_1<A>(
-        mut f: impl FnMut(A) -> T,
-    ) -> impl FnMut(A) -> NeverShortCircuit<T> {
-        move |a| NeverShortCircuit(f(a))
+    pub(crate) const fn wrap_mut_1<A, F>(f: F) -> Wrapped<T, A, F>
+    where
+        F: [const] FnMut(A) -> T,
+    {
+        Wrapped { f, p: PhantomData }
     }
 
     #[inline]
@@ -401,7 +422,8 @@ impl<T> NeverShortCircuit<T> {
 
 pub(crate) enum NeverShortCircuitResidual {}
 
-impl<T> Try for NeverShortCircuit<T> {
+#[rustc_const_unstable(feature = "const_never_short_circuit", issue = "none")]
+impl<T> const Try for NeverShortCircuit<T> {
     type Output = T;
     type Residual = NeverShortCircuitResidual;
 
@@ -415,15 +437,15 @@ impl<T> Try for NeverShortCircuit<T> {
         NeverShortCircuit(x)
     }
 }
-
-impl<T> FromResidual for NeverShortCircuit<T> {
+#[rustc_const_unstable(feature = "const_never_short_circuit", issue = "none")]
+impl<T> const FromResidual for NeverShortCircuit<T> {
     #[inline]
     fn from_residual(never: NeverShortCircuitResidual) -> Self {
         match never {}
     }
 }
-
-impl<T> Residual<T> for NeverShortCircuitResidual {
+#[rustc_const_unstable(feature = "const_never_short_circuit", issue = "none")]
+impl<T> const Residual<T> for NeverShortCircuitResidual {
     type TryType = NeverShortCircuit<T>;
 }
 

library/coretests/tests/array.rs

@@ -724,3 +724,17 @@ fn array_eq() {
     let not_true = [0u8] == [].as_slice();
     assert!(!not_true);
 }
+
+#[test]
+fn const_array_ops() {
+    const fn doubler(x: usize) -> usize {
+        x * 2
+    }
+    const fn maybe_doubler(x: usize) -> Option<usize> {
+        x.checked_mul(2)
+    }
+    assert_eq!(const { std::array::from_fn::<_, 5, _>(doubler) }, [0, 2, 4, 6, 8]);
+    assert_eq!(const { [5, 6, 1, 2].map(doubler) }, [10, 12, 2, 4]);
+    assert_eq!(const { [1, usize::MAX, 2, 8].try_map(maybe_doubler) }, None);
+    assert_eq!(const { std::array::try_from_fn::<_, 5, _>(maybe_doubler) }, Some([0, 2, 4, 6, 8]));
+}

library/coretests/tests/lib.rs

@@ -4,6 +4,7 @@
 #![feature(alloc_layout_extra)]
 #![feature(array_ptr_get)]
 #![feature(array_try_from_fn)]
+#![feature(array_try_map)]
 #![feature(array_windows)]
 #![feature(ascii_char)]
 #![feature(ascii_char_variants)]
@@ -16,6 +17,7 @@
 #![feature(char_internals)]
 #![feature(char_max_len)]
 #![feature(clone_to_uninit)]
+#![feature(const_array)]
 #![feature(const_cell_traits)]
 #![feature(const_cmp)]
 #![feature(const_convert)]